cyr_expire is used to run a number of regular maintenance tasks
on Cyrus databases, specifically:

expire messages from mailboxes, and

expire entries from the duplicate delivery database, and

cleanse mailboxes of partially expunged messages (when using the “delayed” expunge mode), and

remove deleted mailboxes (when using the “delayed” delete mode), and

expire entries from conversations databases.

The expiration of messages is controlled by the
/vendor/cmu/cyrus-imapd/expire mailbox annotation which specifies
the age (in days) of messages in the given mailbox that should be
deleted. A value of 0 means that no expiration is to be performed on
that mailbox.

The value of the /vendor/cmu/cyrus-imapd/expire annotation is
inherited by all children of the mailbox on which it is set, so an
entire mailbox tree can be configured by setting a single annotation on
the root of that tree. If a mailbox does not have a
/vendor/cmu/cyrus-imapd/expire annotation set on it (or does not
inherit one), then no messages are expired from the mailbox.

The annotation can be examined using the info command of
cyradm(8), and modified using the mboxconfig and
setinfo commands of cyradm(8).

Expiration of duplicate delivery database entries for a given mailbox
is also controlled by the /vendor/cmu/cyrus-imapd/expire annotation
which applies to that mailbox. Unlike message expiration, if no
annotation applies to the mailbox then duplicate database entries are
expired using the value given to the -E option.

Expiration of conversations database entries occurs if the
conversations option is present in imapd.conf(5).
Expiration can be disabled using the -c option. The period used to
expire entries is controlled by the conversations_expire_days
option in imapd.conf(5).

cyr_expire reads its configuration options out of the imapd.conf(5) file unless specified otherwise by -C.

Archive non-flagged messages older than archive-duration to the
archive partition, allowing mailbox messages to be split between fast
storage and slow large storage. Only does anything if
archivepartition-* has been set in your config.

Remove previously deleted mailboxes older than delete-duration
(when using the “delayed” delete mode).
The value can be a floating point number, and may have a suffix to
specify the unit of time. If no suffix, the value is number of days.
Valid suffixes are d (days), h (hours), m (minutes) and
s (seconds).

Prune the duplicate database of entries older than expire-duration.
This value is only used for entries which do not have a corresponding
/vendor/cmu/cyrus-imapd/expire mailbox annotation.
Format is the same as delete-duration.

Do not expunge messages even if using delayed expunge mode. This
reduces IO traffic considerably, allowing cyr_expire to be run
frequently to clean up the duplicate database without overloading
the machine.